Hysteresis-Driven CO2 Sequestration Using Ionic Liquids: A CMG Study of the Alida Carbonate Aquifer

Carbon capture and storage (CCS) in carbonate aquifers depends strongly on residual (hysteresis) and dissolution trapping, both of which are controlled by capillarity and wettability. This study evaluates, at reservoir scale, whether injecting an ionic liquid (IL) solution ahead of CO2, which alters interfacial tension (IFT) and wettability, can increase CO2 immobilization. A compositional CMG-GEM model of the brine-saturated Alida Beds (Mission Canyon Formation, Williston Basin, Saskatchewan, Canada) at about 1.2 km depth was used to simulate five years of CO2 injection, limited to 90% of the estimated fracture pressure, followed by 995 years of shut-in; about 1.35 Mt of CO2 was injected in all cases. IL properties were taken from published data for 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and the IL effects were represented through the rock–fluid functions of the rock contacted by the injected IL slug (IL-treated rock): capillary pressure scaled for a 22% IFT reduction and a maximum residual gas saturation raised from 0.40 to 0.42–0.48 to represent a shift toward water-wet conditions. A full CO2 mass balance, mechanism separation runs, and a no-hysteresis control were performed. Hysteresis was the dominant trapping control: without it, only 49.6% of the injected CO2 was immobilized after 1000 years, compared with 93.8% with it, and about 35% of the total immobilization formed after injection stopped. The IFT effect alone changed immobilization by only 0.1 percentage points, whereas the wettability effect reduced mobile CO2 after 1000 years from 7.2% to 3.4–5.8% of the injected CO2 and raised immobilized CO2 after 100 years from 88.4% to 90.5–95.6%. The results indicate that ILs could improve storage security mainly by increasing residual trapping, and that laboratory measurement of IL-induced residual trapping in carbonate rock at storage conditions is the key step needed to confirm this benefit.